what gear/converter combo???

How much street driving will you do and how much gear can you put up with ?

Your 400 has a short stroke/big bore for a bick block and will want to spin some rpms.

If it were me Id run 4.30 gears with a 28" tire and a 9.5" converter with around 3500 rpm stall.

Stay away from the TCI. Id look in to a Turbo Action, Dynamic, ATI or PTC converter.